We are studying the effects of salts and pH on the stability of apo-flavodoxin and some of its fragments. Previously we have demonstrated that the stability of apo-flavodoxin measured by urea denaturation at pH 7 is salt dependent. More recently, we have established that the effect is specific for Na+ salts. This effect has also been found in a fragment of apo-flavodoxin containing residues 1-139. In order to further characterize the physical basis of this cation effect, we are studying the thermodynamics of unfolding by DSC under a variety of ionic conditions. The structure of apo-flavodoxin has been solved recently and we will attempt to simulate the effects of cations on the stability of this protein with algorithms designed to identify cation binding sites and to calculate the energetics of cation binding.